A. L. Chistyakov

Polyhalomethanes combined with aluminum halides as generators of superelectrophiles of a novel type

Abstract Semi-empirical (AM1) and ab initio (MO LCAO SCF-RHF/6–31G) quantum chemical calculations of CCl 4 · n AlCl 3 ( n = 1, 2) and CCl 3 + AlCl 3 systems were performed to elucidate the nature of complexes responsible for the superelectrophilicity of tetrachloromethane in the presence of aluminum chloride. The results of the calculations do not allow us to relate the high activity of these systems with formation of the species Cl 2 CCl + → AlCl 3 . The CCl 4 · n AlCl 3 systems were shown to be generators of the superelectrophiles of a novel type, namely the electron deficient ‘chlorenium’ cations Cl 2 C-Cl + and dications Cl + C-C-Cl + which are different from the known chloronium cations R-Cl + -R. The capability of forming mono-, di- and tridentate ionic complexes, wherein Cl + of the CCl 3 + cation is bonded with one, two or three Cl − of AlCl 4 − , is a peculiarity of the CCl 3 + AlCl 4 − system. The bidentate complex CCl 3 + AlCl 4 − with positive charge 0.54a.u. on the Cl atom (instead of 0.33 a.u. in a free CCl 3 + ) seems to be the best candidate for the role of key superelectrophile in the CCl 4 · n AlCl 3 systems.

Nucleophilic assistance in methane activation by superelectrophiles with halogen-centered cationic sites

Simulation of fragments of potential energy surface for systems CH4 + CBr3+, CH4 + CBr3+AlBr4−, CH4 + CCl3+AlCl4−, and CH4 + CCl3+Al2Cl7− was performed by DFT-B3LYP and DFT-PBE methods. The important role of nucleophilic assistance in methane halogenation by these superelectrophiles was confirmed. These reactions occur with a synchronous hydride transfer from methane to the electrophile within the cyclic transition states in linear C-H-C fragment of the rings and a generation of a C-Hlg bond between the carbon atom of the arising methyl group and the halogen atom of the electrophile. The nucleophilic assistance from the unshared electron pair of this halogen atom provides the lowering of the potential barriers to methane halogenation by complexes CBr3+AlBr4−, CCl3+AlCl4−, and CCl3+Al2Cl7− to the values of the order of 20 kcal mol−1. These essential features of the mechanism of methane halogenation are independent of the halogen nature and are retained on going from the model electrophiles to the real ones.

New Mechanisms of Alkane Activation by Superelectrophiles. MNDO/PM3 Study of Activation of Methane and Propane by Cationic Bromine-Centered Electrophiles

Fragments of the potential energy surface for the systems C3H8 + Br2·AlBr3, CH4 + CBr3+, and CH4 + CBr3+ AlBr4- were simulated by the MNDO/PM3 method with a view to study the mechanisms of reactions of alkanes with new-generation superelectrophiles. Two new mechanisms were revealed for the reaction of alkanes with superelectrophiles. In the three examined systems, weakly bonded adducts are formed, which may be regarded as electrophile “solvates” with alkane molecules. Various ways of transformations of such solvates were examined. Radically new mechanisms were proposed for the bromination of methane with superelectrophiles CBr3+ and CBr3+ AlBr4-. The potential barrier to bromination is reduced to 17-25 kcal/mol due to nucleophilic assistance by lone electron pairs on the bromine atoms. By contrast, the reaction of propane with the Br2·AlBr3 complex is described in terms of the classical Olah scheme.

MNDO/PM3 Study of the Reaction Mechanism between Methane and Complexes Generated in Br2·AlBr3 System

Potential energy surface (PES) of systems AlBr5 and AlBr5 + CH4+ were investigated by MNDO/PM3 method. All the five donor-acceptor complexes Br2·AlBr3 with no barrier add to methane providing multiple adducts with various localization of interactions and with different conformations. However further transformations occur only with adducts of two complexes Br2·AlBr3 possessing considerable ionic character. On the reaction path resulting in CH3Br and HBr as intermediates function bromonium type complexes CH3BrH+·AlBr4- and the intermediates on the path leading to CH2Br2 and H2 are the complexes with 3c-2e bond of H2 quasi-molecule with the C atom of bromomethyl cation H2C(H-H)Br+·AlBr4-. Potential barriers on both reaction paths are about 30 kcal mol-1, and the transition states (TS) are analogous to the classical 3c-2e TS (Olah scheme) with an electrophile attack on a CH bond and to the recently suggested TS with an electrophile attack on an unshared electron pair of the carbon atom in the nonclassical methane H2C(H-H) respectively.

MNDO/3 study of the relative reactivity of ethylene derivatives upon addition to the benzyl radical

The transition states for the addition of a benzyl radical to substituted ethylenes CH2=CHX were determined by the MNDO/3 method, where X=H, CF3, CN, CH3, C4H9, C(CH3)3, CO2CH3, and Si(CH3)3. The activation energies of the forward and back reactions were determined.

Theoretical investigation of the 1,3-migration of hydrogen in the telomerization of ethylene and propylene with methanol

Semiempirical quantum-chemical calculations by the MINDO/3 method have been carried out with complete optimization of the geometry of all the radicals in the molecules which take part in the telomerization of ethylene and propylene with methanol. For each of the telomerization processes the transition states have been determined for three competing reactions: 1,3-H-migration, chain transfer, and chain growth. It is concluded by a comparison of the heat effects and the potential barriers that 1,3-H-migration is possible for the telomerization of propylene, but improbable for the telomerization of ethylene.

Quantum chemical aspects of the 1,3-hydrogen migration in radical telomerization

Conclusions1.The MINDO/3 method was used to study three model competing reactions of the radicals formed upon the telomerization of ethylene and propylene: 1,3-hydrogen migration, chain propagation, and chain transfer.2.1,3-Hydrogen migration is more likely in the telomerization of propylene than in the telomerization of ethylene, in which the activation energy is twice that of the two competing reactions.

Conjugation energy of some boron-containing systems

The conjugation energies of some boron-containing heterocyclic compounds have been calculated. In a number of cases, in spite of the unusual valency angles, within the framework of the MO LCAO method in the π-electron approximation, these systems prove to be stable.